In an automatic soldering apparatus, processing equipment such as a fluxer, a preheater, a wave soldering tank, and a cooler are arranged in series, and a conveyor runs above the processing equipment. While being transported by the conveyor, a printed circuit board undergoes flux application in the fluxer, preheating in the preheater, adhesion of solder in the wave soldering tank, and cooling in the cooler to carry out soldering.
All of the processing equipment in an automatic soldering apparatus affects the quality of soldering, but the wave soldering tank has a particularly great effect. For example, the wave soldering tank must not have pulsing in which the height of spouted solder moves up and down, and oxides which are floating in molten solder must not be enveloped therein.
A wave soldering tank carries out soldering by spouting molten solder upwards and contacting a printed circuit board with the spouted molten solder. For this purpose, a wave soldering tank requires a discharge pump for pumping molten solder and a discharge nozzle for upwardly spouting molten solder which is sent to it by the pump.
The discharge pump and the discharge nozzle are connected by a duct. The discharge pump is provided at one end of the duct, and the discharge nozzle is installed at the other end of the duct. The discharge pump is installed inside a casing formed at the end of the duct. The discharge pump sucks in molten solder and sends the molten solder to the discharge nozzle through the duct, and it spouts molten solder upwards from the discharge nozzle.
Pumps used in wave soldering tanks include impeller pumps and screw pumps. An impeller pump has a large number of plate-shaped blades radially installed on the periphery of a shaft. The casing of an impeller pump is formed at one end of a duct and has roughly the shape of a snail. The shaft of an impeller pump passes through a shaft hole formed in the top of the casing and projects above the surface of molten solder. An inlet of the casing is formed in the bottom of the casing.
As a result of rotation of the large number of blades of an impeller pump, the molten solder between the blades is thrown outwards by centrifugal force and passed through the duct and sent to the discharge nozzle. The impeller pump merely has blades radially installed on a shaft, so it is easy to manufacture and inexpensive. However, with an impeller pump, pulsation of the solder spouted from the discharge nozzle is sometimes experienced. An impeller pump causes the pulsation of molten solder since the molten solder is pumped discontinuously. Namely, in an impeller pump, after molten solder is thrown outwards from between one set of blades, molten solder is thrown outwards from between the next adjoining set of blades. In the period between the two occurrences, there is an instant in which molten solder is not thrown outwards, and this causes pulsation in an impeller pump.
A screw pump has one or more plate-shaped members helically wrapped around the periphery of a shaft. Molten solder rests on and is transported by the helical plate-shaped members. Accordingly, a screw pump does not have discontinuous operation, and theoretically it does not cause pulsations. From long in the past, there have been many proposals in patents and utility models of wave soldering tanks using screw pumps (Patent Documents 1-7).    Patent Document 1: JP S48-19425 Y    Patent Document 2: JP S48-98520 U    Patent Document 3: JP S48-55025 U    Patent Document 4: JP S50-148327 U    Patent Document 5: JP S51-3632 U    Patent Document 6: JP S62-259665 A    Patent Document 7: JP 2005-28446 A